Cover element provided with transparent pressure-sentitive adhesive layer

ABSTRACT

The present invention provides a cover element having a pressure-sensitive adhesive sheet preliminarily laminated thereto, wherein the pressure-sensitive adhesive sheet comprises a pressure-sensitive adhesive layer in which a refractive index adjustment zone having a refractive index greater than that of a base pressure-sensitive adhesive material thereof is formed over a given range from a surface of the pressure-sensitive adhesive layer in a thickness direction thereof, whereby: in a lamination process of a customer which is a supply destination of the cover element, it becomes possible to eliminate a need to distinguish between obverse and reverse sides of the pressure-sensitive adhesive sheet itself; and, when the cover element is bonded to an optical element through the pressure-sensitive adhesive layer, it becomes possible to suppress internal reflection in a laminate formed of these optical elements.

TECHNICAL FIELD

The present invention relates to a cover element provided with atransparent pressure-sensitive adhesive layer. In particular, thepresent invention relates to a cover element provided with a transparentpressure-sensitive adhesive layer usable for bonding a transparentoptical element to another, second, optical element.

BACKGROUND ART

In a display device such as a liquid crystal display device or anorganic EL display device, a pressure-sensitive adhesive is used to bonda transparent optical element, such as a polarizing film, a retardationfilm or a transparent cover element including a cover glass, to anotheroptical element. That is, a pressure-sensitive adhesive layer isdisposed between two optical elements to be bonded together, and thenthe two optical elements are pressed against each other and bondedtogether to thereby form an optical element laminate. In a displaydevice, the optical element laminate configured as above is disposedsuch that the transparent optical element is located on a viewing side.This configuration involves a problem that, when outside light entersfrom the transparent optical element on the viewing side, the enteredlight is reflected at an interface between the pressure-sensitiveadhesive layer and the optical element on a non-viewing side, andreturned to the viewing side. This problem becomes prominentparticularly when an entry angle of outside light is relatively large.

On the other hand, in a touch panel-equipped display device which hasbeen increasingly becoming popular in late years, a transparentelectroconductive layer such as a patterned ITO (Indium Tin Oxide) layeris formed on a surface of a bonding-target optical element to which atransparent optical element is to be bonded. In this type of displaydevice, the problem “poor pattern invisibility” is pointed out that apattern of the transparent electroconductive layer becomes visible fromthe viewing side, by the influence of internal reflection of enteredlight at an interface between a pressure-sensitive adhesive layer andthe transparent electroconductive layer.

In each case, the internal reflection is caused by a difference inrefractive index between the pressure-sensitive adhesive layer and eachof the bonding-target optical element and the transparentelectroconductive layer. JP 4640740 B (Patent Document 1) teaches atechnique for addressing this problem. Specifically, the Patent Document1 discloses a pressure-sensitive adhesive composition capable ofreducing total reflection of light at an interface between a transparentoptical element and a pressure-sensitive adhesive layer and at aninterface between the pressure-sensitive adhesive layer and abonding-target optical element. The composition disclosed in the PatentDocument 1 is allegedly described as having a high refractive index inits dried and/or cured states, wherein it is close to refractive indexesof the transparent optical element and the bonding-target opticalelement body. The teaching of the Patent Document 1 is that apressure-sensitive adhesive layer for bonding two optical elementstogether is entirely formed to have a refractive index close torefractive indexes of the two optical elements.

The technique taught by the Patent Document 1 would be effective insuppressing the interface reflection. On the other hand, there is aproblem that the composition itself becomes costly, because thistechnique is based on the use of a particular monomer component.

JP 5564748 B (Patent Document 2) discloses a refractive index-adjustedpressure-sensitive adhesive which comprises a transparent basepressure-sensitive adhesive material comprised of an acrylic-basedresin, and zirconium oxide or titanium oxide particles having an averagedispersed particle size of 1 nm to 20 nm, wherein the zirconium oxide ortitanium oxide particles are dispersed over the entire thickness of thetransparent base pressure-sensitive adhesive material. In thispressure-sensitive adhesive, the zirconium oxide or titanium oxideparticles as a high refractive index material are mixed with thetransparent base pressure-sensitive adhesive material. Thus, it isconsidered that a refractive index of a layer of the pressure-sensitiveadhesive can be increased in its entirety to thereby suppress theaforementioned interface reflection. However, the technique disclosed inthe Patent Document 2 requires using the high refractive index materialin large amounts. This causes a concern about deterioration inproperties required as a pressure-sensitive adhesive, and a problem ofan increase in cost. Moreover, the high refractive index material usedin the Patent Document 2 is particles of an inorganic material. Thus,there is another problem that the particles are hardly dispersed,causing whitish haze due to light scattering. In this regard, it isconceivable to use particles of an organic material. In this case,however, it becomes difficult to solve a problem of coloration.

With a view to improving the technique disclosed in the Patent Document2, JP 5520752 B (Patent Document 3) proposes coating, with a polymer,metal oxide particles to be dispersed in a pressure-sensitive adhesive.The teaching of the Patent Document 3 is that, although thepressure-sensitive adhesive layer in the Patent Document 2 has a problemof deterioration in adherence property thereof because the metal oxideparticles are exposed to a surface of the pressure-sensitive adhesivelayer, this problem can be solved by coating the metal oxide particleswith a polymer. The technique proposed by the Patent Document 3 couldhave a potential to improve the adherence property of thepressure-sensitive adhesive layer to some extent. However, it cannotsolve most of the remaining problems pointed out in connection with thePatent Document 2. In particular, the technique described in the PatentDocument 3 is based on coating the metal oxide particles with aparticular polymer. Thus, the resulting pressure-sensitive adhesivebecomes more costly than that in the Patent Document 2.

CITATION LIST Patent Document

Patent Document 1: JP 4640740 B

Patent Document 2: JP 5564748 B

Patent Document 3: JP 5520752 B

SUMMARY OF INVENTION Technical Problem

It is a primary object of the present invention to provide a coverelement having a pressure-sensitive adhesive sheet laminated thereto,wherein the pressure-sensitive adhesive sheet comprises apressure-sensitive adhesive layer capable of being easily produced atlow cost and effectively suppressing internal reflection when used forbonding of an optical element laminate.

A pressure-sensitive adhesive sheet having a refractive index adjustmentzone (refractive index adjustment zone-formed pressure-sensitiveadhesive sheet) is offered with two PET separators laminated to oppositesides thereof, so that it is difficult to distinguish between obverseand reverse sides of the pressure-sensitive adhesive sheet itself.Examples of a way to distinguish therebetween include distinguishbetween the obverse and reverse sides based on a difference in peelforce during peel-off of the PET separators. For example, the obverseand reverse sides of the refractive index adjustment zone-formedpressure-sensitive adhesive sheet can be distinguished by configuringthe sheet such that one of the PET separators laminated to one surfaceof the pressure-sensitive adhesive layer defined by the refractive indexadjustment zone can be peeled off by a relatively small peel force(light peelability), and the other PET separator laminated to the othersurface of the pressure-sensitive adhesive layer can be peeled off by arelatively large peel force (heavy peelability). However, a differencein peeling force needs to be set to about 1.5 to 3 times. This leads toa problem of difficulty in adjusting a balance with peel force necessaryas a carrier sheet in a lamination process of a customer which is asupply destination of the refractive index adjustment zone-formedpressure-sensitive adhesive sheet.

Solution to Technical Problem

In brief, in order to solve this problem and achieve the above object,the present invention provides a cover element having apressure-sensitive adhesive sheet preliminarily laminated thereto,wherein the pressure-sensitive adhesive sheet comprises apressure-sensitive adhesive layer in which a refractive index adjustmentzone having a refractive index greater than that of a basepressure-sensitive adhesive material thereof is formed over a givenrange from a surface of the pressure-sensitive adhesive layer in athickness direction thereof, whereby: in a lamination process of acustomer which is a supply destination of the cover element, it becomespossible to eliminate a need to distinguish between obverse and reversesides of the pressure-sensitive adhesive sheet itself; and, when thecover element is bonded to an optical element through thepressure-sensitive adhesive layer, it becomes possible to suppressinternal reflection in a laminate formed of these optical elements.

According to one aspect of the present invention, there is provided acover element provided with a transparent pressure-sensitive adhesivelayer, wherein the pressure-sensitive adhesive layer comprises a baseadhesive zone made essentially of a transparent base pressure-sensitiveadhesive material and formed over a given range from one principalsurface of the pressure-sensitive adhesive layer in a thicknessdirection of the pressure-sensitive adhesive layer, and a transparent,adherent, refractive index adjustment zone formed over a given rangefrom the other principal surface of the pressure-sensitive adhesivelayer in the thickness direction, and wherein the base adhesive zone isin contact with the cover element, and the refractive index adjustmentzone has a refractive index greater than a refractive index of the basepressure-sensitive adhesive material.

Preferably, the refractive index adjustment zone has a thickness of 20nm to 600 nm. In one embodiment of the present invention, the refractiveindex adjustment zone may be formed by dispersing, in apressure-sensitive adhesive material identical to the basepressure-sensitive adhesive material, particles of a high refractiveindex material having a refractive index greater than that of thepressure-sensitive adhesive material, to thereby increase an averagerefractive index of the refractive index adjustment zone. In thisembodiment, the refractive index of the high refractive index materialparticles is preferably in the range of 1.60 to 2.74. Preferably, thehigh refractive index material particles have an average primaryparticle size of 3 nm to 100 nm as measured by TEM observation.Preferably, the high refractive index material is one or more compoundsselected from the group consisting of TiO₂, ZrO₂, CeO₂, Al₂O₃, BaTiO₃,Nb₂O₅ and SnO₂.

In another embodiment of the present invention, the other principalsurface of the pressure-sensitive adhesive layer defined by therefractive index adjustment zone may be formed such that it has a regionon which the high refractive index material particles are partiallyexposed, and a matrix region on which the base pressure-sensitiveadhesive material of the refractive index adjustment zone is partiallyexposed. In this embodiment, the region on which the high refractiveindex material particles are exposed is preferably formed in an arearatio of 30 to 99%. Further, a difference between the refractive indexof the high refractive index material particles and the refractive indexof the base pressure-sensitive adhesive material is preferably in therange of 0.15 to 1.34.

In the present invention, the pressure-sensitive adhesive layerpreferably has a total light transmittance of 80% or more. The highrefractive index material particles may partially exist in the form ofan aggregate arising from aggregation of two or more thereof.

Preferably, the refractive index adjustment zone has a thickness of 20nm to 600 nm. In one embodiment of the present invention, the refractiveindex adjustment zone may be formed by dispersing, in apressure-sensitive adhesive material identical to the basepressure-sensitive adhesive material, particles of a high refractiveindex material having a refractive index greater than that of thepressure-sensitive adhesive material, to thereby increase an averagerefractive index of the refractive index adjustment zone. In thisembodiment, preferably, the refractive index of the basepressure-sensitive adhesive material is set in the range of 1.40 to1.55, and the refractive index of the high refractive index materialparticles is set in the range of 1.60 to 2.74. A bonding surface of therefractive index adjustment zone bonded to an optical element is formedsuch that it has a region where the high refractive index materialparticles are in contact with the optical element, and a matrix regionwhere the pressure-sensitive adhesive material of the refractive indexadjustment zone is in contact with the optical element. In this case,the region where the high refractive index material particles are incontact with the optical element is preferably formed in an area ratioof 30 to 99%. Further, a difference between the refractive index of thehigh refractive index material particles and the refractive index of thebase pressure-sensitive adhesive material is preferably in the range of0.15 to 1.34.

In yet another embodiment of the present invention, the refractive indexadjustment zone may be formed by adding, to a pressure-sensitiveadhesive material identical to the base pressure-sensitive adhesivematerial, an organic material having a refractive index greater thanthat of the pressure-sensitive adhesive material, in the form ofparticle, polymer or oligomer, to thereby increase an average refractiveindex of the refractive index adjustment zone. In the case where thepressure-sensitive adhesive layer having this feature is applied to aconfiguration obtained by forming a transparent electroconductive layeron an optical element, the refractive indexes of the transparentelectroconductive layer, the base pressure-sensitive adhesive materialand the organic material are preferably set, respectively, in the rangeof 1.75 to 2.14, in the range of 1.40 to 1.55 and in the range of 1.59to 2.04. Examples of the high refractive index organic material usableherein include, but are not particularly limited to, a resin having anaromatic ring such as a styrene-based resin, and a resin containing aheteroatom such as a sulfur atom or a nitrogen atom (e.g., a polymercontaining a thiol or triazine ring). Further, examples of the particlesinclude nanometer-size organic nanoparticles and nanometer-sizespherical macromolecules. Preferably, such particles have an averageprimary particle size of 3 nm to 100 nm as measured by TEM observation.

Preferably, the pressure-sensitive adhesive layer preferably has a totallight transmittance of 80% or more. The high refractive index materialparticles may partially exist in the form of an aggregate arising fromaggregation of two or more thereof. In the present invention, therefractive index adjustment zone typically exists with irregular depthsin the thickness direction of the pressure-sensitive adhesive layer.

For example, when the pressure-sensitive adhesive sheet relating to thepresent invention is used to bond a transparent, first optical elementto a second optical element having a transparent electroconductive layerconstituting a touch sensor or the like, the pressure-sensitive adhesivelayer is peeled off from the support, and positionally adjusted suchthat one surface of the pressure-sensitive adhesive layer defined by thetransparent, adherent, refractive index adjustment zone faces thetransparent electroconductive layer and the second optical element, andthe other surface of the pressure-sensitive adhesive layer faces thefirst optical element. Then, the other surface of the pressure-sensitiveadhesive layer is bonded to the first optical element, and therefractive index adjustment zone is brought into contact with both ofthe transparent electroconductive layer and the second optical elementin such a manner as to fill up a stepped space between the transparentelectroconductive layer and the second optical element, wherebyreflected light resulting from reflection of external light enteringthrough the first optical element, at an interface between the baseadhesive zone and the refractive index adjustment zone, and reflectedlight resulting from reflection of the external light at an interfacebetween the refractive index adjustment zone and the transparentelectroconductive layer, are at least partially cancelled out by meansof optical interference.

Thus, when the pressure-sensitive adhesive sheet relating to the presentinvention is used to suppress such internal reflection, it is possibleto at least partially cancel out reflected light resulting fromreflection of external light entering through the first optical element,at an interface between a zone made essentially of the basepressure-sensitive adhesive material and the refractive index adjustmentzone in the pressure-sensitive adhesive layer, and reflected lightresulting from reflection of the external light at an interface betweenthe refractive index adjustment zone and the second optical element, bymeans of optical interference.

Effect of Invention

In the present invention, the refractive index adjustment zone having arefractive index greater than that of the base pressure-sensitiveadhesive material is formed over a given range from the other principalsurface of the pressure-sensitive adhesive layer in the thicknessdirection, so that it becomes possible to form a high refractive indexregion without causing an increase in haze value. Thus, when thetransparent electroconductive layer-equipped cover element using thepressure-sensitive adhesive sheet relating to the present invention isbonded to a second optical element (e.g., patterned transparentelectroconductive film), the refractive index adjustment zone as a highrefractive index region can adjust a refractive index difference withrespect to the second optical element to thereby suppress reflection atthe interface between the pressure-sensitive adhesive layer and thesecond optical element.

In the configuration where a patterned transparent electroconductivelayer is formed on the second optical element, the refractive index ofthe refractive index adjustment zone of the pressure-sensitive adhesivelayer can be adjusted with respect to respective refractive indexes ofthe transparent electroconductive layer and the second optical element,so as to suppress interface reflection. Further, reflected lightreturning toward the first optical element can be significantly reducedby a cancel-out effect based on an inter-reflected light phasedifference among reflected light by the transparent electroconductivelayer, reflected light by the second optical element and reflected lightoccurring inside the pressure-sensitive adhesive layer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1(a) is a sectional view depicting one embodiment of apressure-sensitive adhesive sheet relating to the present invention.

FIG. 1(b) is a sectional view depicting an optical element laminatepresented as one example of a simplest embodiment using apressure-sensitive adhesive sheet according to the present invention (asimplest example of use of a pressure-sensitive adhesive sheet in FIG.1(a)).

FIG. 2 is a sectional view depicting one embodiment of apressure-sensitive adhesive layer for use in the pressure-sensitiveadhesive sheet relating to the present invention.

FIG. 3 is a sectional view depicting an embodiment where thepressure-sensitive adhesive layer 13 depicted in FIG. 2 is applied to aconfiguration in which a patterned transparent electroconductive layeris formed on an optical element.

FIG. 4 is a top plan view depicting a state of a principal surface ofthe pressure-sensitive adhesive layer which is in contact with a secondoptical element.

FIG. 5(a) is a schematic diagram illustrating a step of application of adispersion liquid, in a process for preparing the pressure-sensitiveadhesive layer depicted in FIG. 2.

FIG. 5(b) is a schematic diagram illustrating a step of infiltrationwith high refractive index material particles, in the process forpreparing the pressure-sensitive adhesive layer depicted in FIG. 2.

FIG. 5(c) is a schematic diagram illustrating a step of drying, in theprocess for preparing the pressure-sensitive adhesive layer depicted inFIG. 2.

FIG. 6 is a schematic diagram depicting a configuration of a laminateusing a cover element having a refractive index adjustment zone-formedpressure-sensitive adhesive layered thereon, according one embodiment ofthe present invention.

FIG. 7(a) is a schematic diagram depicting a configuration of a coverelement for use in Inventive Examples and Comparative Examples.

FIG. 7(b) is a schematic diagram depicting a configuration of anothercover element for use in Inventive Examples and Comparative Examples.

FIG. 8(a) depicts a configuration of a refractive index adjustmentzone-formed pressure-sensitive adhesive (A1) for use in InventiveExamples and Comparative Examples.

FIG. 8(b) depicts a configuration of another refractive index adjustmentzone-formed pressure-sensitive adhesive (B1) for use in InventiveExamples and Comparative Examples.

FIG. 9(a) depicts a configuration of a patterned transparentelectroconductive film for use in Inventive Examples and ComparativeExamples.

FIG. 9(b) depicts a configuration of another patterned transparentelectroconductive film for use in Inventive Examples and ComparativeExamples.

FIG. 10(a) depicts a configuration of an optical element laminate inInventive Example 1.

FIG. 10(b) depicts a configuration of an optical element laminate inInventive Example 2.

FIG. 11(a) depicts a configuration of an optical element laminate inComparative Example 1.

FIG. 11(b) depicts a configuration of an optical element laminate inComparative Example 2.

FIG. 12 is a 20000× SEM photograph presenting a surface state of arefractive index adjustment zone of a pressure-sensitive adhesive layerproduced in one Inventive Example.

FIG. 13(a) is a 30000× TEM cross-sectional photograph presenting a highrefractive index material particle distribution in a refractive indexadjustment zone of a pressure-sensitive adhesive layer obtained inanother Inventive Example.

FIG. 13(b) is a 30000× TEM cross-sectional photograph presenting a highrefractive index material particle distribution in a refractive indexadjustment zone of a pressure-sensitive adhesive layer obtained inanother Inventive Example.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will now be described inconnection with the figures. FIG. 1(a) is a sectional view depicting oneembodiment of a pressure-sensitive adhesive sheet relating to thepresent invention. FIG. 1(b) is a sectional view depicting an opticalelement laminate 1 presented as one example of a simplest embodimentusing a pressure-sensitive adhesive sheet according to the presentinvention (a simplest example of use of a pressure-sensitive adhesivesheet in FIG. 1(a)). Referring to FIG. 1(a), a pressure-sensitiveadhesive sheet S relating to the present invention comprises: anoptically transparent pressure-sensitive adhesive layer 3; a firstsupport S1 composed of a separator laminated to one principal surface ofthe pressure-sensitive adhesive layer 3; and a second support S2composed of a separator laminated to the other principal surface of thepressure-sensitive adhesive layer 3. Referring to FIG. 1(b), the opticalelement laminate 1 comprises an optically transparent, first opticalelement 2, and a second optical element 4 bonded to the first opticalelement 2 through an optically transparent pressure-sensitive adhesivelayer 3. This pressure-sensitive adhesive layer 3 is obtained by peelingoff the supports S1, S2 from the pressure-sensitive adhesive sheet Sdepicted in FIG. 1(a), wherein the first and second optical elements arelaminated to the exposed opposite surfaces of the pressure-sensitiveadhesive layer 3. The transparent, first optical element 2 may becomposed of an optical film for use in an optical display device, suchas a polarizing film or a retardation film, or a transparent coverelement for use in an optical display device, such as a viewing-sidecover glass. The first optical element 2 and the second optical element4 are bonded, respectively, to a first principal surface 5 and a secondprincipal surface 6 of the pressure-sensitive adhesive layer 3.

The transparent pressure-sensitive adhesive layer 3 comprises a baseadhesive zone 3 a made essentially of a base pressure-sensitive adhesivematerial, and a refractive index adjustment zone 3 b having a refractiveindex greater than that of the base adhesive zone 3 a. Preferably, therefractive index of the base pressure-sensitive adhesive material of thebase adhesive zone 3 a is close to the refractive index of the firstoptical element 2.

The base pressure-sensitive adhesive material is not particularlylimited, as long as it is a transparent adherent material usable inoptical applications. For example, it is possible to use one or moreappropriately selected from the group consisting of an acrylic-basedpressure-sensitive adhesive, a rubber-based pressure-sensitive adhesive,a silicone-based pressure-sensitive adhesive, a polyester-basedpressure-sensitive adhesive, a urethane-based pressure-sensitiveadhesive, an epoxy-based pressure-sensitive adhesive, and apolyether-based pressure-sensitive adhesive. From a viewpoint oftransparency, processability, durability, etc., it is preferable to usean acrylic-based pressure-sensitive adhesive. As the basepressure-sensitive adhesive material, the above pressure-sensitiveadhesives may be used independently or in the form of a combination oftwo or more of them. An acrylic-based polymer to be used as a basepolymer of an acrylic-based pressure-sensitive adhesive is preferably,but not particularly limited to, a homopolymer or copolymer of monomerscomprising a primary component consisting of (meth)acrylic acid alkylester. It should be noted that the term “(meth)acrylic” is used hereinto mean either one or both of “acrylic” and “methacrylic”, and thisdefinition is also applied to the following description. In thisspecification, the term “acrylic-based polymer” is used to mean that itmay include the above (meth)acrylic acid alkyl ester, and any othermonomer copolymerizable with the (meth)acrylic acid alkyl ester.Generally, the refractive index of the base pressure-sensitive adhesivematerial is in the range of 1.40 to 1.55.

Although a thickness of the pressure-sensitive adhesive layer 3 is notparticularly limited, it is set, typically, in the range of 5 μm to 500μm, preferably, in the range of 5 μm to 350 μm, more preferably, in therange of 5 μm to 250 μm. In the pressure-sensitive adhesive layer 3, athickness of the refractive index adjustment zone 3 b is set,preferably, in the range of 20 nm to 600 nm, more preferably, in therange of 20 nm to 300 nm, furthermore preferably, in the range of 20 nmto 200 nm. A boundary between the refractive index adjustment zone 3 band the base adhesive zone 3 a is formed as an irregular undulatingcurve. In the present invention, the thickness of the refractive indexadjustment zone 3 b is determined by averaging a plurality ofmeasurement values of a depth of the undulation. A thickness of the baseadhesive zone 3 a is derived by subtracting the thickness of therefractive index adjustment zone 3 b from the thickness of thepressure-sensitive adhesive layer 3. As measured according to JIS K7361,a total light transmittance of the entire pressure-sensitive adhesivelayer 3 is 80% or more, preferably, 90% or more. It is preferable toachieve a higher total light transmittance of the pressure-sensitiveadhesive layer 3. Further, a haze value of the pressure-sensitiveadhesive layer 3 is preferably 1.5% or less, more preferably, 1% orless.

The refractive index adjustment zone 3 b can be formed, for example, byapplying, by a given amount, a solution of a resin material having arefractive index greater than that of the base pressure-sensitiveadhesive material, onto one surface of a pressure-sensitive adhesivelayer formed of the base pressure-sensitive adhesive material (thislayer will hereinafter be also referred to as “base pressure-sensitiveadhesive material layer”), and then subjecting the resultingpressure-sensitive adhesive layer to drying. Examples of the resinmaterial usable for this purpose include a pressure-sensitive adhesivecomposition described in the Patent Document 1. Alternatively, it ispossible to employ a technique of: dispersing, in the form of a solid,an organic material having a refractive index greater than that of thebase pressure-sensitive adhesive material, such as styrene oligomer, ina dispersion medium to prepare a dispersion liquid; applying thedispersion liquid to a surface of a base pressure-sensitive adhesivematerial layer; and subjecting the resulting pressure-sensitive adhesivelayer to drying. However, in the present invention, it is preferable toemploy a technique of causing particles of a high refractive indexmaterial to infiltrate into the base pressure-sensitive adhesivematerial layer from one surface thereof, whereby the high refractiveindex material particles are dispersed in a region of the basepressure-sensitive adhesive material layer adjacent to the one surface,as described below in connection with FIG. 2.

With reference to FIG. 2, a configuration of a pressure-sensitiveadhesive layer 13 according to one embodiment of the present inventionwill be described in detail below.

As with the pressure-sensitive adhesive layer 3 in the embodimentdepicted in FIG. 1, the pressure-sensitive adhesive layer 13 depicted inFIG. 2 according to one embodiment of the present invention has a firstprincipal surface 15 and a second principal surface 16, and comprises abase adhesive zone 13 a made essentially of a base pressure-sensitiveadhesive material, and a refractive index adjustment zone 13 b having arefractive index greater than that of the base adhesive zone 13 a. Inthis example, the refractive index adjustment zone 13 b is formed suchthat it contains particles 17 of a high refractive index material whichare caused to infiltrate into a base pressure-sensitive adhesivematerial layer from the second principal surface 16, and dispersed inthe base pressure-sensitive adhesive material layer, over a given depthin a thickness direction of the base pressure-sensitive adhesivematerial layer, to thereby have a refractive index greater than that ofthe base adhesive zone 13 a.

Preferably, a refractive index of the high refractive index materialparticles 17 in the refractive index adjustment zone 13 b is in therange of 1.6 to 2.7. Preferably, a difference between the refractiveindex of the high refractive index material particles and the refractiveindex of the base pressure-sensitive adhesive material is in the rangeof 0.2 to 1.3. In the case where the refractive index adjustment zone isformed by impregnating a part of the base pressure-sensitive adhesivematerial layer with an organic material having a refractive indexgreater than that of the base pressure-sensitive adhesive material, adifference between the refractive index of the organic material and therefractive index of the base pressure-sensitive adhesive material ispreferably set in the range of 0.1 to 0.6. Examples of a high refractiveindex material usable in this example where high refractive indexmaterial particles are used in the refractive index adjustment zoneinclude TiO₂, ZrO₂, CeO₂, Al₂O₃, BaTiO₂, Nb₂O₅ and SnO₂, and one or morecompounds selected from them can be used to form the high refractiveindex material particles 17. The high refractive index materialparticles 17 may be set to have an average primary particle size of 3 nmto 100 nm, and distributed in the refractive index adjustment zone 13 bin an individually dispersed state or in a partially aggregated state.As described in connection with FIG. 1, a boundary between therefractive index adjustment zone 13 b and the base adhesive zone 13 a isformed as an irregular undulating curve. In a thickness measurement forthe refractive index adjustment zone 13 b, a depth at each of aplurality of measurement positions in a region where 90% of the highrefractive index material particles 17 exist is determined as athickness measurement value, and the measurement values at the pluralityof measurement positions are averaged to obtain a thickness of therefractive index adjustment zone 13 b.

FIG. 3 is a sectional view depicting an embodiment in which thepressure-sensitive adhesive layer 13 depicted in FIG. 2 is applied to aconfiguration obtained by forming a patterned transparentelectroconductive layer 7 such as a patterned ITO film, on apressure-sensitive adhesive layer-side surface of the second opticalelement 4, so as to make up a touch panel sensor. In this case, examplesof the second optical element 4 may include a glass substrate of adisplay panel, for example, in a liquid crystal display device or anorganic EL display device.

As depicted in FIG. 3, the principal surface 16 of the refractive indexadjustment zone 13 b of the pressure-sensitive adhesive layer 13 isbonded to respective pressure-sensitive adhesive layer-side surfaces ofthe second optical element 4 and the transparent electroconductive layer7 in such a manner as to fill up a stepped space between the secondoptical element 4 and the transparent electroconductive layer 7. FIG. 4is a top plan view depicting a state of the principal surface 16 of thepressure-sensitive adhesive layer 13 in contact with the second opticalelement 4. As depicted in FIG. 4, a microstructure of the principalsurface 16 is formed in a sea-island structure where the high refractiveindex material particles 17 are dispersed in a matrix 18 of the basepressure-sensitive adhesive material in the form of islands. In acontact surface of the pressure-sensitive adhesive layer 13 with thesecond optical element 4, there are a region where the basepressure-sensitive adhesive material is in contact with the secondoptical element 4 and a region where the high refractive index materialparticles 17 are in contact with the second optical element 4.Preferably, a ratio of an area of the high refractive index materialparticles 17 to a total area of the high refractive index materialparticles 17 and the base pressure-sensitive adhesive material at theabove position is set in the range of 30 to 99%.

The area ratio is calculated by measuring an area of the high refractiveindex material particles 17 in each of a plurality of square regionshaving a side length of 10 μm to 200 μm to obtain a ratio of the area ofthe high refractive index material particles 17 to the entire area ofthe square region, and averaging the area ratios measured in theplurality of square regions.

FIGS. 5(a), 5(b) and 5(c) schematically depict a process of producingthe pressure-sensitive adhesive layer 13 in FIG. 2. First of all, adispersion liquid 19 obtained by dispersing the above high refractiveindex material particles 17 in a dispersion medium, and a basepressure-sensitive adhesive material layer 20, are preliminarilyprepared. Then, as depicted in FIG. 5(a), the dispersion liquid 19 isapplied to a surface of the base pressure-sensitive adhesive materiallayer 20. Thus, the surface of the base pressure-sensitive adhesivematerial layer 20 is swelled by the dispersion medium of the dispersionliquid 19, and, in this process, the high refractive index materialparticles 17 in the dispersion liquid 19 infiltrate into the basepressure-sensitive adhesive material layer 20 in the thicknessdirection. This state is depicted in FIG. 5(b). Subsequently, theresulting base pressure-sensitive adhesive material layer 20 is dried tovaporize the dispersion medium of the dispersion liquid 19 to obtain thepressure-sensitive adhesive layer 13 depicted in FIG. 2, i.e., arefractive index adjustment zone-formed pressure-sensitive adhesivelayer. This state is depicted in FIG. 5(c).

An infiltration depth of the high refractive index material particles 17with respect to the base pressure-sensitive adhesive material layer 20is determined by a relationship between the base pressure-sensitiveadhesive material and the dispersion medium of the dispersion liquid 19.The dispersion medium may be appropriately selected to enable theinfiltration depth (thickness of the refractive index adjustment zone)to become the aforementioned value.

EXAMPLES

Although the present invention will be further described below, based onexamples, it is to be understood that the present invention is notlimited to the following examples. In the following description aboutthe examples, part(s) and % are on the basis of weight (by weight), and,unless otherwise stated below, conditions of leaving a sample to standat room temperature (room temperature standing) are 23° C. and 65% RH inall the cases.

FIG. 6 depicts a laminate 21 using a cover element having a reflectiveindex adjustment zone-formed pressure sensitive adhesive layeredthereon, according to one embodiment of the present invention. In thisembodiment, one surface of the reflective index adjustment zone-formedpressure sensitive adhesive (IM sublayer-formed pressure sensitiveadhesive 23) defined by a reflective index adjustment zone thereof isattached to a cover element (glass/resin) 22. Then, the other surface ofthe IM sublayer-formed pressure sensitive adhesive 23 on a side oppositeto the reflective index adjustment zone is attached to one surface of apatterned transparent conductive film 24 on the side of a patternedelectroconductive (ITO) layer. The other surface of the patternedtransparent conductive film 24 on a side opposite to theelectroconductive (ITO) layer can be attached to an image display (LCD,OLED) 25 such as an LCD panel or an OLED panel).

An image display device described below comprises the cover element ofthe present invention. Although the following description will be madeby taking a liquid crystal display device as an example, the presentinvention is also applicable to any type of image display devicesrequiring a transparent electroconductive layer-equipped cover element.Specific examples of an image display compatible with the transparentelectroconductive layer-equipped cover element of the present inventioninclude a liquid crystal display, an electroluminescence (EL) display, aplasma display (PD), and a field emission display (FED). The imagedisplay device in the present invention comprises the transparentelectroconductive layer-equipped cover element of the present invention,and the remaining configuration is the same as that of a conventionalimage display device.

A liquid crystal cell drive mode is not particularly limited, but any ofvarious heretofore-known modes may be used. Examples of the drive modeinclude a twisted nematic (TN) mode, a super twisted nematic (STN) mode,an electrically-controlled birefringence (ECB) mode, a verticalalignment (VA) mode, an in-plane switching (IPS) mode, a fringe fieldswitching (FFS) mode, optically compensated bend (OCB) mode,hybrid-aligned nematic (HAN) mode, a surface-stabilized ferroelectricliquid crystal (SSFLC) mode, and an anti-ferroelectric liquid crystal(AFLC) mode. Among them, from a viewpoint of being capable of minimizingbrightness change/color change due to viewing angle, it is preferable touse the in-plane switching (IPS) mode. Further, as necessary, a colorfilter, a black matrix or the like may be provided on either substrateof a liquid crystal cell.

Cover Lens Element

A 0.65 mm-thick alkali-free glass (refractive index: 1.53) was used as aglass cover lens. FIG. 7(a) depicts a cover element 26 presenting aconfiguration of such a glass cover element. A 0.7 mm-thicksurface-hardened acrylic plate (trade name “ACRYLITE® MR200”,manufactured by Mitsubishi Rayon Co., Ltd., surface (hardened layer)refractive index: 1.53) was used as a plastic cover lens. FIG. 7(b)depicts a cover element 27 presenting a configuration of such a plasticcover element. A numerical value described in each layer in FIGS. 7(a)and 7(b) denotes a refractive index. Similarly, a numerical valuedescribed in each layer in the following figures (FIGS. 8(a) to 11(b))denotes a refractive index, unless a unit is indicated.

The cover element may be formed, for example, of a glass material or atransparent resin substrate, and may be formed in a single-layerstructure or a composite structure composed of a number of members. Thethickness of the cover element may be in the range of 0.05 to 2.00 mm,preferably, 0.1 to 1.3 mm, particularly preferably, 0.2 to 1.1 mm Incase of using a glass material having a thickness of 0.2 mm or less, itis possible to obtain a substrate having excellent bendability, butthere is a risk of development of a crack and breakage. Thus, in orderto prevent the risk, it is preferable to provide a resin layer on one oreach of opposite sides of the glass material. The substrate may bepartly or entirely formed in a curved or rounded shape.

In the case where the cover element is formed of a glass material, it ispossible to select a glass plate having excellent strength andtransmittance, such as soda glass, alkali-free glass, borosilicate glassor aluminosilicate glass. When a glass plate having excellent strengthis selected, it can be thinned. In particular, chemically toughenedglass (aluminosilicate, soda lime) is preferably used from a viewpointof excellent compressive strength.

Examples of a raw material usable for the transparent resin substrateinclude: polyester-based resin, such as PET or PEN; cyclo-olefin basedresin, such as COP or COC; polyolefin based resin, such as PE, PP,polystyrene or EVA; vinyl-based resin; polycarbonate-based resin;urethane-based resin; polyamide-based resin; polyimide-baaed resin;acrylic-based resin; epoxy-based resin; polyarylate-based resin;polysulfone-based resin; silsesquioxane-based resin; andtriacetylcellulose (TAC). In order to avoid the occurrence of colorationand color unevenness due to retardation, an optically isotropicsubstrate is preferable. Examples of a recommended substrate for theoptically isotropic substrate include cyclo-olefin based resin,polycarbonate-based resin and polyarylate-based resin.

The transparent resin substrate may be formed as a composite material byadding and dispersing an organic or inorganic material in the aboveresin. A form of the material may be, but not particularly limited to, alayer form, a particle form or a fiber form. For example, as alayer-form or particle-form material, it is possible to use layer-formclay mineral (clay) or inorganic particles of metal oxide such as silicaor titanium oxide, or the like. From a viewpoint of transparency, thelayer-form or particle form material preferably has a particle size of100 nm or the less. Examples of the fiber-form material include anorganic-based material such as nylon-based resin oracetylcellulose-based resin, and a glass-based material. From aviewpoint of transparency, the fiber-form material is preferably ananofiber having a diameter of 100 nm or the less. With a view toimprovement in dispersibility and compatibility with respect to theresin, each of the above materials may be surface-modified. Such anorganic or inorganic-based material is added and dispersed in the resinto form a composite material, so that it becomes possible to obtain ahighly functional resin substrate improved in various physicalproperties such as tensile strength, elastic modulus and heat distortiontemperature, while maintaining transparency.

The cover element may have a functional film on an outer side (viewingside) thereof as viewed from the image display. Examples of thefunctional layer include a hard coat (HC) layer, an anti-reflectionlayer, an anti-fouling layer, an anti-static layer, and a treated layerfor the purpose of diffusion or anti-glare. The functional layer may beformed by arbitrarily combining two or more of them. Further, anultraviolet absorbing function may be given to the cover element and/orthe functional layer.

A shatterproof protective film may be laminated to the outer or innerside of the cover element. The shatterproof film may have the above oneor more functional layers. In order to avoid the occurrence ofcoloration and color unevenness due to retardation, it is preferable touse an optically isotropic substrate (e.g., a non-stretched cyclo-olefinpolymer film, or a polycarbonate film based on a casting process).Further, at an arbitrary position between the cover element and theimage display, a retardation film (λ/4 wavelength plate) for coping withsunglasses may be disposed. Preferably, the retardation film (λ/4wavelength plate) is disposed such that a slow axis thereof is at 45degrees with respect to an absorption axis of a viewing-side polarizingplate of the image display.

The cover element may be provided with a decorative layer. Thedecorative layer is formed using by colored ink containing a resinbinder, and pigment or dye as a colorant. The decorative layer may beformed in a single-or multi-layer structure by means of screen printing,offset printing, gravure printing or the like, and a thickness of theprinted layer is generally set in the range of about 0.5 to 50 μm.Further, in order to express metal luster, a layer composed of a metalthin film formed by vapor deposition or sputtering. The decorative layermay be any one of the opposite surfaces of the cover element, or may beformed on a film such as the aforementioned shatterproof film.

Production of Base Pressure-Sensitive Adhesive Materials Production ofAcrylic Oligomer

60 weight parts of dicyclopentanyl methacrylate (DCPMA), 40 weight partsof methyl methacrylate (MMA), 3.5 weight parts of α-thioglycerol as achain transfer agent, and 100 weight parts of toluene as apolymerization medium were put into a four-neck flask, and stirred in anitrogen atmosphere at 70° C. for 1 hour. Then, 0.2 weight parts of2,2′-azobisisobutyronitrile as a polymerization initiator was put intothe four-neck flask to induce a reaction at 70° C. for 2 hours, followedby a further reaction at 80° C. for 2 hours. Then, the resultingreaction solution was placed in an atmosphere at 130° C. to remove thetoluene, the chain transfer agent and unreacted monomers therefrom bydrying to thereby obtain an acrylic-based polymer in a solid form. Theacrylic-based polymer obtained in this manner was named as“acrylic-based polymer (A-1)”. This acrylic-based polymer (A-1) had aweight-average molecular weight (Mw) of 5.1×10³.

Production of Base Pressure-Sensitive Adhesive Material A

0.035 weight parts of a photopolymerization initiator (trade name:“IRGACURE 184”, manufactured by BASF SE.) and 0.035 weight parts of aphotopolymerization initiator (trade name “IRGACURE 651” manufactured byBASF SE.) were added to a monomer mixture of 68 weight parts of2-ethylhexyl acrylate (2EHA), 14.5 weight parts of N-vinyl-2-pyrrolidone(NVP) and 17.5 weight parts of 2-hydroxyethyl acrylate (HEA), and thenthe resulting monomer mixture was exposed to ultraviolet light in anitrogen atmosphere in such a manner as to be partiallyphotopolymerized, thereby obtaining a partially polymerized producthaving a polymerization rate of about 10 weight % (acrylic-based polymersyrup).

Then, 5 weight parts of the acrylic-based polymer (A-1), 0.15 weightparts of hexanediol diacrylate (HDDA) and 0.3 weight parts of a silanecoupling agent (trade name: “KBM-403”, manufactured by Shin-EtsuChemical Co., Ltd.) were added to and uniformly mixed with theacrylic-based polymer syrup obtained in the above manner to obtain anacrylic-based pressure-sensitive adhesive composition. The acrylic-basedpressure-sensitive adhesive composition was applied onto arelease-treated surface of a separator film (trade name: “DIAFOILMRF#38”, manufactured by Mitsubishi Plastics, Inc.) in such a mannerthat a thickness thereof after being formed as a base pressure-sensitiveadhesive material layer becomes 250 μm, thereby forming apressure-sensitive adhesive composition layer. Then, a separator film(trade name: “DIAFOIL MRF#38”, manufactured by Mitsubishi Plastics,Inc.) was attached onto a surface of the pressure-sensitive adhesivecomposition layer in such a manner that a release-treated surface ofthis separator film faces the applied layer. In this way, the appliedlayer of the monomeric components is blocked from oxygen. Subsequently,the pressure-sensitive adhesive composition layer was photocured byirradiation with ultraviolet light under conditions including anilluminance of 5 mW/cm² and a light intensity of 1,500 mJ/cm², to form alayer of a base pressure-sensitive adhesive martial A.

Production of Base Pressure-Sensitive Adhesive Material B

28.5 weight parts of 2-ethylhexyl acrylate (2EHA), 28.5 weight parts ofisostearyl acrylate (ISTA), 22 weight parts of isobornyl acrylate, 20weight parts of 4-hydroxybutyl acrylate (4HBA), and two types ofphotopolymerization initiators: 0.05 weight parts of aphotopolymerization initiator (trade name: “IRGACURE 184”, manufacturedby BASF SE.) and 0.05 weight parts of a photopolymerization initiator(trade name: “IRGACURE 651”, manufactured by BASF) were mixed together,and the resulting monomer mixture was exposed to ultraviolet light in anitrogen atmosphere in such a manner as to be partiallyphotopolymerized, thereby obtaining a partially polymerized product(acrylic-based polymer syrup) having a polymerization rate of about 10weight %.

Then, 0.3 weight parts of hexanediol diacrylate (HDDA) and 0.3 weightparts of a silane coupling agent (trade name: “KBM-403”, manufactured byShin-Etsu Chemical Co., Ltd.) were added to and evenly mixed with theacrylic-based polymer syrup obtained in the above manner to obtain anacrylic-based pressure-sensitive adhesive composition. The acrylic-basedpressure-sensitive adhesive composition was applied onto arelease-treated surface of a separator film (trade name: “DIAFOILMRF#38”, manufactured by Mitsubishi Plastics, Inc.) in such a mannerthat a thickness thereof after being formed as a base pressure-sensitiveadhesive material layer becomes 125 μm, thereby forming apressure-sensitive adhesive composition layer. Then, a separator film(trade name: “DIAFOIL MRF#38”, manufactured by Mitsubishi Plastics,Inc.) was attached onto a surface of the pressure-sensitive adhesivecomposition layer in such a manner that a release-treated surface ofthis separator film faces the applied layer. In this way, the appliedlayer of the monomer components is blocked from oxygen. Subsequently,the pressure-sensitive adhesive composition layer was photocured byirradiation with ultraviolet light under conditions including anilluminance of 5 mW/cm² and a light intensity of 1,500 mJ/cm², to form alayer of a base pressure-sensitive adhesive material B.

The base pressure-sensitive adhesive material needs to be excellent inoptical transparency, adhesiveness, and cohesive property(processability). As the base pressure-sensitive adhesive material, itis possible to use one or more appropriately selected from the groupconsisting of an acrylic-based pressure-sensitive adhesive, arubber-based pressure-sensitive adhesive, a silicone-basedpressure-sensitive adhesive, a polyester-based pressure-sensitiveadhesive, an urethane-based pressure-sensitive adhesive, an epoxy-basedpressure-sensitive adhesive, and a polyether-based pressure-sensitiveadhesive. Although the thickness of the base pressure-sensitive adhesivematerial layer is not particularly limited, it is set typically in therange of 5 to 500 μm, preferably in the range of 10 to 350 μm, morepreferably in the range of 25 to 250 μm.

Various additives may be added to the base pressure-sensitive adhesivecomposition for use in the present invention. For example, it ispreferable to add any of various silane coupling agents so as to improveadhesion under high-temperature and high-humidity conditions. It is alsopreferable to add a cross-linking agent to impart a cohesion forceassociated with durability of a pressure-sensitive adhesive. Further, itis possible to use, as needed basis, viscosity modifier, releaseregulator, tackifier, plasticizer, softener, filler composed of aninorganic powder or the like, colorant (pigment, dye, etc.), pH adjuster(acid or base), antioxidant, rust inhibitor, and ultraviolet absorber.

A sheet-shaped pressure-sensitive adhesive (base pressure-sensitiveadhesive material layer) may be formed by, but not particularly limitedto, a technique of applying the base pressure-sensitive adhesivecomposition onto any of various substrates (a separator film, atransparent resin film, etc.), and drying the applied basepressure-sensitive adhesive composition by a dryer such as a thermaloven, to vaporize a solvent or the like; a technique of subjecting anactive energy ray-curable composition applied on any of varioussubstrates to curing such as ultraviolet irradiation; or a technique ofsubjecting a thermally curable resin composition applied on any ofvarious substrates to thermal curing.

Alternatively, the base pressure-sensitive adhesive material layer maybe formed by a technique of applying an active energy ray-curablecomposition directly onto a transparent electroconductive layer formedon a cover element in a given pattern (a sensor) and subjecting theapplied composition to curing such as ultraviolet irradiation; or atechnique of subjecting a thermally curable resin composition applieddirectly onto the transparent electroconductive layer to thermal curing.

Production of Refractive Index Adjustment Sub-Layer-FormedPressure-Sensitive Adhesives Case of Using Base Pressure-SensitiveAdhesive Material A & High Refractive Index Material NanoparticleDispersion Liquid Case of Using Base Pressure-Sensitive AdhesiveMaterial A & Nanoparticle Dispersion Liquid

A 250 μm-thick base pressure-sensitive adhesive material layer A(refractive index: 1.49) having opposite surfaces each protected by aPET separator sheet was prepared, and one of the PET separator sheetswas peeled off. A coating liquid (dispersion medium: ethanol, particleconcentration: 1.2 weight %, transmittance of dispersion liquid: 82%;manufactured by CIK Nanotech Co., Ltd.) containing zirconia particles(ZrO₂, refractive index: 2.17, average primary particle size: 20 nm), asa dispersion liquid containing high refractive index material particles,was applied to the exposed surface of the base pressure-sensitiveadhesive material layer by using a bar coater RDS No. 5 in such a manneras to form a refractive index adjustment zone having a thickness of 20nm to 200 nm, and dried in a drying oven at 110° C. for 180 seconds.Then, a PET separator sheet (75 μm) serving as a support (backing) wasattached onto a surface of the resulting pressure-sensitive adhesivelayer, from which the zirconia (ZrO₂) particles were dispersed, tothereby obtain a refractive index adjustment zone-formedpressure-sensitive adhesive (A1). An average primary particle size ofthe zirconia particles was measured by TEM observation. FIG. 8(a)depicts a configuration of the refractive index adjustment zone-formedpressure-sensitive adhesive (A1).

Other Examples

Using the following base pressure-sensitive adhesive material layers andhigh refractive index material nanoparticle dispersion liquid,refractive index adjustment zone-formed pressure-sensitive adhesive (B1)was produced in the same manner as that in the above case. The materialsused were the base pressure-sensitive adhesive material B (refractiveindex: 1.48) and ZrO₂ nanoparticle dispersion liquid (dispersion medium:ethanol, particle size: 20 nm).

A list of properties of pressure-sensitive adhesives used in evaluationsis presented in the following Table. FIG. 8(b) depicts the refractiveindex adjustment zone-formed pressure-sensitive adhesive (B1).

TABLE 1 Base Pressure- Refractive Index Sensitive Adhesive AdjustmentZone- Material Formed Pressure- Thickness High Refractive SensitiveAdhesive Type μm Index Material Production Method Refractive index (A)250 Zirconium oxide Applying and drying ethanol adjustment zone-Refractive (average particle dispersion liquid (solid formed pressure-index: size 20 nm) content: 1.2 wt %, sensitive adhesive 1.49transmittance of dispersion (A1) liquid: 82%) to form a rlective indexadjustment zone having a thickness of 20 to 200 nm Refractive index (Ba)125 Zirconium oxide Applying and drying ethanol adjustment zone-Refractive (average particle dispersion liquid (solid formed pressure-index: size 20 nm) content: 1.2 wt %, sensitive adhesive 1.48transmittance of dispersion (B1) liquid: 82%) to form a rlective indexadjustment zone having a thickness of 20 to 200 nm Properties ofRefractive Index Adjustment Zone in Laminate Thickness of HighRefractive Adhesive Index Force at Layer in Surface of Average cross-Refractive Refractive Index Interfacial sectional Index Adjustment Zone-Refractive TEM Total Light Adjustment Formed Pressure- Index observationTransmittance Zone Sensitive Adhesive n_(D) [nm] [%] Haze Value [N/25mm] Refractive index 1.67 150 92.3 0.7 17 adjustment zone- formedpressure- sensitive adhesive (A1) Refractive index 1.76 120 92.3 0.5 12adjustment zone- formed pressure- sensitive adhesive (B1)

The refractive index adjustment zone is made of a suitable one selectedfrom a plurality of materials each having a refractive index greaterthan a refractive index of the base pressure-sensitive adhesivematerial. From a viewpoint of compatibility with the basepressure-sensitive adhesive material (bleed-out under low temperatures,risk of segregation under high temperatures) and durability under hightemperatures, it is preferable to use an inorganic high refractive indexmaterial. It is possible to use one or more compounds selected from thegroup consisting of TiO₂, ZrO₂, CeO₂, Al₂O₃, BaTiO₂, Nb₂O₅ and SnO₂.Preferably, the high refractive index material particles have an averageprimary particle size of 3 nm to 100 nm. The thickness of the refractiveindex adjustment zone is preferably in the range of 20 nm to 600 nm,more preferably, in the range of 20 nm to 300 nm, furthermorepreferably, in the range of 20 nm to 200 nm.

Production of Transparent Electroconductive Layer Using COP as Substrate

A coating liquid obtained by adding 0.07 parts of a plurality of 3μm-diameter particles (trade name: “SSX105”, manufactured by SekisuiJushi Corp.) to 100 parts of binder resin (trade mark “UNIDIC RS29-120”,manufactured by DIC Corp.) was applied to each of opposite surfaces of a100 μm-thick cyclo-olefin polymer film (trade name: “ZEONOA ZF16”,manufactured by Nippon Zeon Co., Ltd., in-plane birefringence: 0.0001)by using a bar coater, and dried in an oven at 80° C. for 1 minute.Then, each surface of the resulting film was irradiated with ultravioletlight (high-pressure mercury vapor lamp) with an integrated lightintensity of 300 mJ/cm², thereby forming a film having anti-blockinglayers on respective opposite surfaces thereof (this film willhereinafter be referred to as “COP substrate”). Then, a refractive indexadjuster (trade name: “OPSTAR KZ6661”, manufactured by JSR Corp.) wasapplied to one surface of the COP substrate by using a bar coater, and,after being dried in an oven at 80° C. for 1 minute, irradiated withultraviolet light (high-pressure mercury vapor lamp) with an integratedlight intensity of 300 mJ/cm², so that it was formed as a 100 nm-thickrefractive index adjustment layer having a refractive index of 1.65.Then, a 23 nm-thick indium tin oxide (ITO) layer serving as atransparent electroconductive layer was deposited on the surface of therefractive index adjustment layer of the COP substrate, by using awinding type sputtering apparatus. After forming a photoresist film on apart of a surface of the transparent electroconductive layer, theresulting film was immersed in an aqueous hydrogen chloride solution(hydrochloric acid concentration: 5 weight %) at 25° C., for 1 minute,to subject the transparent electroconductive layer to etching. Throughthe etching, a patterned transparent electroconductive film (1) having anon-removed portion of the transparent electroconductive layercorresponding to an electrode wiring pattern (ITO 32 in FIG. 9(a)), anda removed portion of the transparent electroconductive layer (opening),was obtained. FIG. 9(a) depicts a laminate 31 presenting a configurationof the patterned transparent electroconductive film (1).

Production of Transparent Electroconductive Layer Using PET as Substrate

A coating liquid obtained by adding 0.1 parts of a plurality of 3μm-diameter particles (trade name: “SSX105”, manufactured by SekisuiJushi Corp.) to 100 parts of binder resin (trade mark “UNIDIC RS29-120”,manufactured by DIC Corp.) was applied to one of opposite surfaces of a50 μm-thick PET film (trade name: “LUMIRROR: U40”, manufactured by TorayIndustries Inc.) by using a bar coater, and dried in an oven at 80° C.for 1 minute. Then, the surface of the resulting film was irradiatedwith ultraviolet light (high-pressure mercury vapor lamp) with anintegrated light intensity of 300 mJ/cm², thereby forming a 1.5 μm-thickanti-blocking layer on one surface of the PET film (the resulting filmwill hereinafter be referred to as “PET substrate”). Then, athermally-curable resin consisting of melamine resin: alkyd resin: andorganic silane condensate (weight ratio of 2:2:1) was applied to theother surface of the PET substrate on the opposite side of the coatedsurface, and cured to form a 30 nm-thick refractive index adjustmentlayer having a refractive index 1.54. Then, a 23 nm-thick indium tinoxide (ITO) layer serving as a transparent electroconductive layer wasdeposited on the surface of the refractive index adjustment layer 37 byusing a winding type sputtering apparatus. After forming a photoresistfilm on a part of a surface of the transparent electroconductive layer,the resulting film was immersed in an aqueous hydrogen chloride solution(hydrochloric acid concentration: 5 weight %) at 25° C., for 1 minute,to subject the transparent electroconductive layer to etching. Throughthe etching, a patterned transparent electroconductive film (2) having anon-removed portion of the transparent electroconductive layercorresponding to an electrode wiring pattern (ITO 36 in FIG. 9(b)), anda removed portion of the transparent electroconductive layer (opening),was obtained. FIG. 9(b) depicts a laminate 35 presenting a configurationof the patterned transparent electroconductive film (2).

The transparent electroconductive layer is preferably made of a materialhaving transparency (invisibility) in addition to electroconductiveproperty, and capable of being patterned A material forming thetransparent electroconductive layer is not particularly limited, and itis possible to use a metal oxide of at least one metal selected from thegroup consisting of indium, tin, zinc, gallium, antimony, titanium,silicon, zirconium, magnesium, aluminum, gold, silver, copper, palladiumand tungsten. The metal oxide may further contain a metal atom presentedin the above group as necessary. For the transparent electroconductivelayer, any of various shapes such as a comb-like shape, a stripe shapeand a diamond shape may be employed depending on the intended use. Forexample, it is preferable to use indium oxide (ITO) containing tinoxide, tin oxide containing antimony, or the like. Among them, ITO isparticularly preferable. The ITO may be crystalline ITO or may beamorphous ITO. For example, the crystalline ITO can be obtained bysubjecting ITO to sputtering under high temperatures or by furtherheating the amorphous ITO. Preferably, ITO contains 80 to 99 weight % ofindium oxide and 1 to 20 weight % of tin oxide.

Although a thickness of the transparent electroconductive layer is notparticularly limited, it is preferably set to 7 nm or more, morepreferably, 12 to 200 nm, furthermore preferably, 12 to 100 nm,particularly preferably, 18 to 70 nm. If the thickness of thetransparent electroconductive layer is less than 7 nm, the transparentelectroconductive layer is not evenly attached in plane, so that it islikely that a resistance value in plane becomes unstable, or a desiredresistance value fails to be obtained. On the other hand, if thethickness of the transparent electroconductive layer is greater than 200nm, the transparent electroconductive layer tends to undergodeterioration in productivity, increase in cost, and deterioration inoptical properties.

A method for forming the transparent electroconductive layer is notparticularly limited, but any of various heretofore-known processes maybe employed. Specific examples thereof may include vacuum vapordeposition, sputtering, and ion plating. Among them, an appropriateprocess may be selectively employed according to a required thickness.

Between the electroconductive layer and the cover element or between theelectroconductive layer and the pressure-sensitive adhesive, anundercoat layer or an overcoat layer may be provided. The coat layer maybe formed in a single-layer structure or a two or more-layer, i.e.,multi-layer, structure. The coat layer may have a refractive indexadjustment function. Preferably, the coat layer has a refractive indexequal to or less than that of the electroconductive layer. The coatlayer may additionally have a gas barrier function and/or an anti-rustfunction.

The transparent electroconductive layer may comprise a metal nanowire ora metal mesh. The metal nanowire means an electroconductive substancewhich is made of a metal and formed in a needle-like or thread-likeshape having a nanometer size in diameter. The metal nanowire may belinear or may be curved. When a transparent electroconductive layercomposed of metal nanowires is used, the metal nanowires are formed intoa mesh shape. Thus, even in the case where an amount of metal nanowiresis relatively small, it is possible to form a good electrical conductionpath and obtain a transparent electroconductive film having a relativelysmall electrical resistance. Further, the metal nanowires are formedinto a mesh shape, so that openings are formed between mesh intervals.This makes it possible to obtain a transparent electroconductive filmhaving a high light transmittance.

Any appropriate metal may be used as a metal forming the metal nanowire,as long as the metal has high electroconductivity. Examples of the metalforming the metal nanowire include silver, gold, copper, and nickel.Further, it is possible to use a material obtained by subjecting each ofthe metals to plating (such as gold plating). Among them, from aviewpoint of electroconductivity, silver, copper, or gold is preferable,and silver is more preferable.

The transparent electroconductive layer containing a metal mesh isobtained by forming a thin metal wire into a lattice pattern, on theaforementioned substrate laminate. The metal mesh may be made of thesame metal as that forming the aforementioned metal nanowire. Thetransparent electroconductive layer containing the metal mesh can beformed by any appropriate method. For example, the transparentelectroconductive layer can be obtained by applying a photosensitivecomposition (composition for forming a transparent electroconductivelayer) containing a silver salt onto the substrate laminate, and thensubjecting the resulting substrate laminate to light exposure anddeveloping to form the thin metal wire into a given pattern.

Production of Laminate of Cover Lens Element and Pressure-SensitiveAdhesive (Inventive Examples and Comparative Examples) Inventive Example1

An optical element laminate depicted in FIG. 10(a) was produced.Specifically, the optical element laminate was produced by: peeling offthe PET separator sheet on one surface of the refractive indexadjustment zone-formed pressure-sensitive adhesive (A1) defined by thebase pressure sensitive adhesive material A; attaching the exposedsurface to one surface of the glass cover lens (cover element 26) insuch a manner that the refractive index adjustment zone defines an outersurface (the other surface) of the refractive index adjustmentzone-formed pressure-sensitive adhesive (A1) on a side opposite to theglass cover lens, to form a pressure-sensitive adhesive layer-attachedcover lens (A); peeling off the PET separator sheet on the surface ofthe formed pressure-sensitive adhesive layer-attached cover lens (A)defined by the refractive index adjustment zone; and attaching theexposed surface onto one surface of the transparent electroconductivelayer of the patterned transparent electroconductive film (1) (laminate31) in such a manner that the refractive index adjustment zone (highrefractive index material particle-containing zone) of thepressure-sensitive adhesive layer-attached cover lens (A) is broughtinto contact with the non-removed portion and the removed portion of thetransparent electroconductive layer on the substrate. Further, forreflectance measurement, an evaluation black PET film 39 was attached tothe other surface of the laminate 31 on a side opposite to therefractive index adjustment zone (high refractive index materialparticle-containing zone).

Inventive Example 2

An optical element laminate depicted in FIG. 10(b) was produced. InInventive Example 2, the pressure-sensitive adhesive to be laminated toa cover element was changed to the refractive index adjustmentzone-formed pressure-sensitive adhesive (B1). Then, the optical elementlaminate was produced by: peeling off the PET separator sheet on onesurface of the refractive index adjustment zone-formedpressure-sensitive adhesive (B1) defined by the base pressure sensitiveadhesive material B; attaching the exposed surface to one surface of theplastic cover lens (cover element 27) in such a manner that therefractive index adjustment zone defines an outer surface (the othersurface) of the refractive index adjustment zone-formedpressure-sensitive adhesive (B1) on a side opposite to the cover lens,to form a pressure-sensitive adhesive layer-attached cover lens (B);peeling off the PET separator sheet on the surface of the formedpressure-sensitive adhesive layer-attached cover lens (B) defined by therefractive index adjustment zone; and attaching the exposed surface ontoone surface of the transparent electroconductive layer of the patternedtransparent electroconductive film (2) (laminate 35) in such a mannerthat the refractive index adjustment zone (high refractive indexmaterial particle-containing zone) of the pressure-sensitive adhesivelayer-attached cover lens (B) is brought into contact with thenon-removed portion and the removed portion of the transparentelectroconductive layer on the substrate. Further, for reflectancemeasurement, an evaluation black PET film 39 was attached to the othersurface of the laminate 35 on a side opposite to the refractive indexadjustment zone (high refractive index material particle-containingzone).

Comparative Example 1

An optical element laminate depicted in FIG. 11(a) was produced.Specifically, except that the refractive index adjustment zone-formedpressure-sensitive adhesive (A1) to be laminated was changed to the basepressure-sensitive adhesive material A devoid of the refractive indexadjustment zone, a pressure-sensitive adhesive-attached cover lens (C)was produced in the same manner as that in Inventive Example 1.

Comparative Example 2

An optical element laminate depicted in FIG. 11(b) was produced.Specifically, except that the refractive index adjustment zone-formedpressure-sensitive adhesive (B1) to be laminated was changed to the basepressure-sensitive adhesive material B devoid of the refractive indexadjustment zone, a pressure-sensitive adhesive-attached cover lens (D)was produced in the same manner as that in Inventive Example 2.

A list of Inventive and Comparative Examples and a result of thereflectance measurements are presented in the following Table.

TABLE 2 Pressure-Sensitive Adhesive laminated to RemovedElectroconductive Layer Non-removed portion of portion of Type ofrefractive transparent transparent index adjustment electroconductivelayer electroconductive Pattern Invisibility zone-formed Reflectionlayer Reflectance Color Compre- Optical pressure-sensitive Reflectancesuppressin Reflectance difference difference hensive Laminate adhesiveThickness (Y %) rate (Y %) (%) value (ΔE * ab) evaluation InventiveCover element Pressure-sensitive 200 5.1 −0.5 5.0 0.1 1.8 ◯ Example 1laminate (A) adhesive (1) Inventive Cover element Pressure-sensitive 1255.4 −0.2 5.4 0.0 0.7 ⊚ Example 2 laminate (B) adhesive (2) ComparativeCover element Pressure-sensitive 200 5.7 — 5.4 0.2 3.6 Δ Example 1laminate (C) adhesive A Comparative Cover element Pressure-sensitive 1255.6 — 5.5 0.1 3.1 Δ Example 2 laminate (D) adhesive B

Evaluation Method Measurement of Weight Average Molecular Weight (Mw) ofAcryl-Based Polymer

A weight average molecular weight of the produced acryl-based polymerwas measured by gel permeation chromatography (GPC).

-   -   Apparatus: HLC-8220 GPC manufactured by TOSOH CORPORATION    -   Column: Sample column; TSKguardcolumn Super HZ-H (one column)        and TSKgel Super HZM-H (two columns), manufactured by TOSOH        CORPORATION    -   Reference column; TSKgel Super H-RC (one column), manufactured        by TOSOH CORPORATION    -   Flow rate: 0.6 mL/min    -   Injection amount: 10 μL    -   Column temperature: 40° C.    -   Eluent: THF    -   Concentration of injected sample: 0.2 weight %    -   Detector: differential refractometer

The weight average molecular weight was calculated in terms ofpolystyrene.

Observation of Surface State of Pressure-Sensitive Adhesive Layer

In each Inventive Example, the surface of the pressure-sensitiveadhesive layer having the high refractive index material particles wasobserved using a field emission scanning electron microscope (FE-SEM) atan acceleration voltage of 2 kV, and at respective magnifications: 500times, 2,000 times, 5,000 times and 20,000 times. A SEM photograph at amagnification of 20,000 times is presented in FIG. 8. The SEM photographshows that the high refractive index material particles are uniformlydispersed.

Observation of Gradation-Processed Microstructure

In two Inventive Examples, a cross-section adjacent to the surface ofthe pressure-sensitive adhesive layer having the high refractive indexmaterial particles was observed using a transmission electron microscope(TEM) at a magnification of 30,000 times. Results of the observation arepresented in FIGS. 13(a) and 13(b). FIG. 13(a) shows that the highrefractive index material particles are approximately uniformlydistributed over approximately the entire thickness of the refractiveindex adjustment zone, whereas FIG. 13(b) shows that a distributiondensity of the high refractive index material particles in thepressure-sensitive adhesive layer is maximized at one surface of thepressure-sensitive adhesive layer, and gradually reduced toward theother surface in a thickness direction of the pressure-sensitiveadhesive layer.

Average Interfacial Refractive Index

An average interfacial refractive index of the pressure-sensitiveadhesive layer obtained in each of Inventive Examples and an averageinterfacial refractive index of the base pressure-sensitive adhesivematerial layer obtained in each of Comparative Examples were calculatedby measuring refractive indexes in the sodium D-lines (589 nm) using aspectroscopic ellipsometer (“EC-400” manufactured by JA. Woolam Co.). Ineach of the pressure-sensitive adhesive layers of Inventive andComparative Examples, the separator sheets on the opposite surfaces ofthe pressure-sensitive adhesive layer were peeled off, and a black platewas laminated to a non-particle infiltration-side one of the surfaces.In this state, an average interfacial refractive index at the othersurface, i.e., the particle infiltration-side surface, was measured. Onthe other hand, in each of the pressure-sensitive adhesive layers (basepressure-sensitive adhesive material layers) of Comparative Examples,two separator sheets on respective opposite surfaces of thepressure-sensitive adhesive layer were peeled off, and then a blackplate was laminated to one of the surfaces. In this state, an averageinterfacial refractive index at the other surface of thepressure-sensitive adhesive layer was measured.

Measurement of Thickness of Refractive Index Adjustment Zone (Sublayer)

A cross-section of the pressure-sensitive adhesive layer in a depthdirection was adjusted to perform TEM observation. Based on theresulting TEM image (direct magnification: 3,000 to 30,000 times), athickness of the refractive index adjustment zone was measured. Thethickness of the refractive index adjustment zone was determined as anaverage value of undulation of the interface between the base adhesivezone (sublayer) and the refractive index adjustment zone. In a situationwhere it was difficult to identify the interface between the baseadhesive zone and the refractive index adjustment zone, an interfacialTEM image was subjected to binary image processing using imageprocessing software (ImageJ), and a depth in a region where 90% of thenanoparticles exists was determined as the thickness of the refractiveindex adjustment zone.

Area Ratio of High Refractive Index Material Particles

The particle infiltration-side surface of the pressure-sensitiveadhesive layer was observed using FE-SEM at an acceleration voltage of 2kV, and at respective magnifications: 500 times, 2,000 times and 5,000times. The resulting interfacial SEM image was subjected to binary imageprocessing using image processing software (ImageJ) to measure an areaof the high refractive index material particles in a rectangular regionhaving a long side length of 23 μm and a short side length of 18 μm soas to calculate an area ratio (%) of the high refractive index materialparticles to the entire rectangular region.

Total Light Transmittance & Haze Value

In each of the pressure-sensitive adhesive sheets obtained in Inventiveand Comparative Examples, a particle infiltration-side one of theseparator sheets was peeled off, and the resulting exposed surface wasattached to a glass slide (trade name: “ShiroKenma No. 1”, manufacturedby Matsunami Glass Ind., Ltd., thickness: 0.8 to 1.0 mm, total lighttransmittance: 92%, haze: 0.2%). Then, the other separator sheet wasfurther peeled off to produce a test piece having a three-layeredstructure of the base adhesive sublayer, the refractive index adjustmentsublayer, and the glass slide. On the other hand, in each of thepressure-sensitive adhesive sheets obtained in Comparative Examples, oneof the separator sheets was peeled off, and the resulting exposedsurface was attached to a glass slide (trade name: “ShiroKenma No. 1”,manufactured by Matsunami Glass Ind., Ltd., thickness: 0.8 to 1.0 mm,total light transmittance: 92%, haze: 0.2%). Then, the other separatorsheet was further peeled off to prepare a test piece having atwo-layered structure of the base pressure-sensitive adhesive materiallayer and the glass slide. For each of the test pieces, a total lighttransmittance and a haze value in a visual light range were measuredusing a haze meter (device name: HM-150, manufactured by Murakami ColorResearch Laboratory Co., Ltd).

Adhesive Force During 180-Degree Peeling (Adhesive Force with Respect toGlass Plate During 180-Degree Peeling

A piece having a length of 100 mm and a width of 25 mm was cut from eachof the pressure-sensitive adhesive sheets obtained in Inventive andComparative Examples. Then, in each of the cut pieces of Inventive andComparative Examples, a non-particle infiltration-side one of theseparator sheets was peeled off, and a PET film (trade name: “LUMIRRORS-10”, manufactured by TORAY Industries Inc., thickness: 25 μm) wasattached (lined) to the exposed surface. On the other hand, in each ofthe cut pieces of Comparative Examples 1 and 2, one of the separatorsheets was peeled off, and a PET film (trade name: “LUMIRROR S-10”,manufactured by TORAY Industries Inc., thickness: 25 μm) was attached(lined) to the exposed surface. Subsequently, the other separator sheetwas peeled off, and then the cut piece was press-bonded to a glass plate(trade name: “Soda-Lime Glass #0050”, manufactured by Matsunami GlassInd., Ltd.) as a test plate, under press-bonding conditions: 2 kgroller; and one stroke, to produce a sample having a three-layeredstructure of the test plate. Each of the obtained samples was subjectedto an autoclave treatment (50° C., 0.5 MPa, 15 minutes), and thensubjected to cooling in an atmosphere at 23° C. and 50% R.H., for 30minutes. After the cooling, the pressure-sensitive adhesive sheet (thepressure-sensitive adhesive layer and one PET film) was peeled off fromthe test plate to measure an adhesive force (N/25 mm) during 180-degreepeeling, using a tension tester (device name: Autograph, manufactured byShimadzu Corp.) according to JIS Z0237, in an atmosphere at 23° C. and50% R.H., under conditions including a tension rate of 300 mm/minute anda peeling angle of 180 degrees. Further, in each of Inventive andComparative Examples, a pressure-sensitive adhesive sheet of the basepressure-sensitive adhesive material before the infiltration of the highrefractive index material particles was prepared, and an adhesive forceduring 180-degree peeling was measured in the same manner as above.

Transmittance of High Refractive Index Particle-Containing DispersionLiquid

A transmittance of the high refractive index particle-containingdispersion liquid was measured by a photoelectrometer (AC-114manufactured by Optima Inc.) using a 530 nm filter. On the assumptionthat a transmittance of the dispersion medium itself is 100%, atransmittance (%) of the dispersion liquid used in Inventive andComparative Examples was calculated.

Measurement of Reflection Suppression Rate (Color Difference Value) andReflection Hue

A sample for reflectance measurement was prepared such that one of theopposite surfaces of each of the optical element laminates in Inventiveand Comparative Examples was used as a reflectance measuring surface,and a single-sided pressure-sensitive adhesive black PET sheet(PET75NBPET38, manufactured by Lintec Corporation) was attached to theother surface. A reflectance (Y value) of each of the optical elementlaminates on the side of the reflectance measuring surface, and areflection hue (L*, a*, b* values: CIE 1976), were measured by areflection spectrophotometer (U4100, manufactured by HitachiHigh-Technologies Corp.). The measurement was performed at respectivepositions corresponding to an etched area and a non-etched area of thetransparent electroconductive layer. That is, in the etched area(opening) of the transparent electroconductive layer, a reflectance atan interface between the refractive index adjustment zone of thepressure-sensitive adhesive layer and the substrate of the opticalelement laminate was measured. Further, in the non-etched area(patterned portion) of the transparent electroconductive layer, areflectance at an interface between the refractive index adjustment zoneof the pressure-sensitive adhesive layer and the transparentelectroconductive layer was measured.

For each of the etched area and the non-etched area, a reflectionsuppression rate was calculated based on the following formula. In thefollowing formula, “reflectance (%) in case devoid of the particles”means a reflectance of each of the optical element laminates inComparative Examples (using no particle). That is, the reflectionsuppression rate is an index indicating how much the reflectance can bereduced by providing the refractive index adjustment zone.

Reflection suppression rate (%)=reflectance (%)−reflectance (%) in casedevoid of the particles

A reflection hue improvement rate was measured for each of the etchedarea and the non-etched area by deriving a difference in color value(ΔL*, Δa*, Δb*), and calculating a color-difference value (ΔE*ab) basedon the following formula:

Color-difference value (ΔE*ab)=[(ΔL*)̂2+(Δa*)̂2+(Δb*)̂2]̂(1/2)

That is, the color-difference value (ΔE*ab) is an index indicating adifference in color between the etched area and the non-etched area.

Determination on Pattern Invisibility

Evaluation on pattern invisibility was determined based on a differencein reflected color between a non-removed portion of theelectroconductive layer and a removed portion of the electroconductivelayer. When the reflectance difference (%) was equal to or greater than1.0%, the laminate was evaluated as ×. On the other hand, in a situationwhere the reflectance difference (%) was less than 1.0%, the laminatewas evaluated as ⊚ when the color difference value was less than 1.0, orevaluated as ∘ when the reflectance difference (%) was in the range of1.0 to less than 3.0, or evaluated as Δ when the reflectance difference(%) was equal to or greater than 3.0.

As can be seen from the measurement result depicted in Table 2, thereflection suppression rates in Inventive Examples 1 and 2 using thetransparent electroconductive layer-equipped cover lenses (A) and (B)were in the range of −0.5% to −0.2%. That is, reflection was suppressedby the refractive index adjustment zone-formed pressure-sensitiveadhesives comprised in the pressure-sensitive adhesive-attached coverlenses (A) and (B), to provide an improvement effect. The aforementioneddetermination on pattern invisibility was performed to evaluate theimprovement effect in detail. As a result, the color difference valuewas in the range of 0.7 to 1.8, i.e., a good result could be obtained.On the other hand, in Comparative Examples 1 and 2 using thepressure-sensitive adhesive-attached cover lenses (C) and (D) devoid ofthe refractive index adjustment zone-formed pressure-sensitive adhesive,reflection was not suppressed, so that no improvement effect could beobserved. Specifically, the color difference value was in the range of3.1 to 3.6, i.e., a good result could not be obtained. This shows thatthe pressure-sensitive adhesive-attached cover lenses (A) and (B) inInventive Examples are capable of effectively suppressing reflection.

INDUSTRIAL APPLICABILITY

As mentioned above, the present invention employs a pressure-sensitiveadhesive layer for bonding a first optical element to a second opticalelement, wherein the refractive index adjustment zone having arefractive index greater than a refractive index of the basepressure-sensitive adhesive material is formed over a given range from asurface of the pressure-sensitive adhesive layer on the side of thesecond optical element, in the thickness direction of thepressure-sensitive adhesive layer. Thus, it becomes possible to suppressthe situation where internal reflections of external light are returnedthrough the first optical element. The present invention can be appliedto an optical display device, such as a liquid crystal display deviceand an organic EL display device. In particular, the present inventioncan be advantageously applied to a touch panel type display devicehaving a touch sensor.

LIST OF REFERENCE SIGNS

-   S: pressure-sensitive adhesive sheet-   S1, S2: support (backing)-   1: optical element laminate-   2: first optical element-   3, 13: transparent pressure-sensitive adhesive layer-   3 a, 13 a: base adhesive zone-   3 b, 13 b: refractive index adjustment zone-   4: second optical element-   7: transparent electroconductive layer-   17: high refractive index material particle-   19: dispersion liquid-   20: base pressure-sensitive adhesive material layer-   21: laminate-   22, 26, 27: cover element-   23: refractive index adjustment zone-formed pressure-sensitive    adhesive-   24: transparent electroconductive film-   25: image display-   31, 35: substrate-   32, 36: ITO layer-   33, 37: refractive index adjustment layer-   34: COP substrate-   38: PET substrate-   39: evaluation black PET

1. A cover element provided with a transparent pressure-sensitiveadhesive layer, wherein the pressure-sensitive adhesive layer is made ofa transparent base pressure-sensitive adhesive material and is asingle-layer having two principal surfaces opposite to each other, thepressure-sensitive adhesive layer comprises a base adhesive zone madeessentially of a transparent base pressure-sensitive adhesive materialand formed over a given range from one principal surface of thepressure-sensitive adhesive layer in a thickness direction of thepressure-sensitive adhesive layer, and a transparent, adherent,refractive index adjustment zone formed over a given range from theother principal surface of the single-layered pressure-sensitiveadhesive layer in the thickness direction, and and wherein the baseadhesive zone is in contact with the cover element, and the refractiveindex adjustment zone is configured for including a material having arefractive index greater than a refractive index of the basepressure-sensitive adhesive material in the single-layeredpressure-sensitive adhesive layer.
 2. The cover element as recited inclaim 1, which is a glass substrate or a transparent resin substrate. 3.The cover element as recited in claim 1, wherein the refractive indexadjustment zone has a thickness of 20 nm to 600 nm.
 4. The cover elementas recited in claim 1, wherein the refractive index adjustment zone isformed by dispersing, in a pressure-sensitive adhesive materialidentical to the base pressure-sensitive adhesive material, particles ofa high refractive index material having a refractive index greater thanthat of the pressure-sensitive adhesive material, to thereby increase anaverage refractive index of the refractive index adjustment zone.
 5. Thecover element as recited in claim 4, wherein the refractive index of thehigh refractive index material particles is in the range of 1.60 to2.74.
 6. The cover element as recited in claim 4, wherein the highrefractive index material particles have an average primary particlesize of 3 nm to 100 nm as measured by TEM observation.
 7. The coverelement as recited in claim 4, wherein a difference between therefractive index of the high refractive index material particles and therefractive index of the base pressure-sensitive adhesive material is inthe range of 0.15 to 1.34.
 8. The cover element as recited in claim 4,wherein the high refractive index material is one or more compoundsselected from the group consisting of TiO2, ZrO2, CeO2, Al2O3, BaTiO3,Nb2O5 and SnO2.
 9. The cover element as recited in claim 1, wherein therefractive index adjustment zone is formed by adding, to apressure-sensitive adhesive material identical to the basepressure-sensitive adhesive material, an organic material having arefractive index greater than that of the pressure-sensitive adhesivematerial, in the form of particle, polymer or oligomer, to therebyincrease an average refractive index of the refractive index adjustmentzone.
 10. The cover element as recited in claim 9, wherein therefractive index of the base pressure-sensitive adhesive material is inthe range of 1.40 to 1.55, and the refractive index of the organicmaterial is in the range of 1.59 to 2.04.
 11. The cover element asrecited in claim 1, wherein the pressure-sensitive adhesive layer has atotal light transmittance of 80% or more.
 12. The cover element asrecited in claim 4, wherein the high refractive index material particlespartially exist in the form of an aggregate arising from aggregation oftwo or more thereof.
 13. The cover element as recited in claim 4,wherein the refractive index adjustment zone exists with irregulardepths in the thickness direction of the pressure-sensitive adhesivelayer.